fix doxygen brief in subzero header files

src/IceTargetLoweringX8664.h

 //===- subzero/src/IceTargetLoweringX8664.h - lowering for x86-64 -*- C++ -*-=//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
-/// This file declares the TargetLoweringX8664 class, which implements the
+/// \brief Declares the TargetLoweringX8664 class, which implements the
 /// TargetLowering interface for the X86 64-bit architecture.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef SUBZERO_SRC_ICETARGETLOWERINGX8664_H
 #define SUBZERO_SRC_ICETARGETLOWERINGX8664_H
 
 #include "IceAssemblerX8664.h"
 #include "IceCfg.h"
 #include "IceGlobalContext.h"
@@ skipping 75 matching lines @@
   }
 
 private:
   ENABLE_MAKE_UNIQUE;
 
   explicit TargetHeaderX8664(GlobalContext *Ctx) : TargetHeaderLowering(Ctx) {}
 };
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICETARGETLOWERINGX8664_H